I've owned my 69 since 1978 when water got in the trunk back then I pulled the soft plugs in the lower 1/4's. The car has it's original sheet metal no rust that I can see in the drop offs. Since one can't see down there I was wondering if spraying rust perventer was a good idea.

In a word,YES! Except you did not mention which one or kind. I just posted a day or two ago about a product call Waxoyl I think under my post on "Nagging ?". I also have some spray cans of cosmoline (look online) One drawback, it has an odor for awhile. I would not use anything like the black undercoat or heavy caked on type stuff as moisture can get trapped between the undercoat and panel causing what you are trying to prevent. The products I mention are clear or opaque, thin and adhere well and can be done by a DIY some are in spray aerosol with long tube with a head that sprays out 3 ways to do things like frame rails and tight, unseen places. Prevention is worth a pound of rust,as they say.

Vince/others,Nothing wrong with POR-15 products IMO, just that it is actually a paint that can be put over rusted areas and it cures by the moisture in the air, from what I understand or at least the product I have used. Here is a link to Waxoyl. http://www.waxoyl-usa.com/products/corrosion-prevention Should have had it in my earlier post(lazy, I guess). Looks like Waxoyl (according to the site)will not sell to private buyers and Centers that apply are only in the upper East Coast, as of now. Google shopping shows products available. I learned about product from being a M-B Tech. Used more so in Europe. I am not an authority on paints or rust proofing, so perhaps some of our "body" guys will chime in. I was only sharing what I have had experience with and feel are preferable and superior to the heavier black rust proofing products. There may be a similar product to Waxoyl that can be purchased by a DIY. Eastwood ,I believe have products. You may talk with a reputable shop and thy could possibly have the means to buy the Waxoyl product for you. Another product I have no experience, but have heard reccomended is called Dinitrol (UK) here is one link:http://www.ebay.com/itm/like/321310150227?lpid=82 Might just want to Google "rust prevention,or proofing,or corrosion,or protection" Hope this is of some use.

Go to your local jobber and buy cavity waxes/internal coatings and an application wand. SEM, 3M(Rust Fighter I) and Transtar all have them, Wurth is another good product. I don't use POR products on any job. If you navigate the aforementioned company sites, all the info is available.

Rust Fighter I is just a cavity wax with wicking properties, not a paint or converter. Paints like SEM Rust Shield can be applied first, then followed with cavity wax. The condition of pinchwelds, amount of rust between them and ipotential to pop out is the concern. A solid, pampered car should not rot before you do.